Brian Tyler. He's scoring everything else Zimmer isn't.
But seriously, either somebody the director has worked for (we don't know the director yet), or maybe it will be like Williams -- the name is so attached to the film, the new director will just seek Conti out (if he even wants to do it or is free to do it).
"The film will focus on a man following in the footsteps of his grandfather, Apollo Creed, and getting a mentor in the now-retired Rocky Balboa."
What might have been a better concept: an historical, decades-spanning period-piece in which the young Apollo Creed reaches manhood and comes up through the boxing ranks. But there's no role for (elderly) Rocky in that saga, so Stallone probably wouldn't like it.